The bobbin winds really loose
You need to keep tension on the thread spool as you wind the bobbin to avoid this from happening. Also, is there a little tension device to take the thread around between the spool and the bobbin winder? Some machines have a little silver button tensioner purely for bobbin winding to keep the thread flowing smoothly.
But I always do the following anyway just to ensure a smooth bobbin. Take the thread from your spool, through the eyelet or tensioner, then back to the bobbin and put the tail end up through a hole in the top of the bobbin. Now put the bobbin onto the winder and click it against the stop. Place the curved handle of a pair of scissors onto the top of the thread spool and apply some gentle pressure to stop the spool from bouncing and jumping while winding off. Keep doing this through the winding process.
Start the bobbin winding mechanisim, its a button on my Janome 6500, yours might be a little different. HOLD the thread tail until you've got coverage over the whole bobbin area catching the starting point. Stop, trim the thread tail off top of the bobbin with scissors, then restart winding again. Watch as the thread winds and give it a nudge with your finger tip to the top or bottom so the bobbin winds evenly across the whole spool's width.
Best analogy here is when you wind the garden hose onto the reel, if it goes on all over the place you never get the whole length on neatly, but if you wind it on neatly in an even tight coil across the spool, then back again, then repeat, you get a tidy hose. But just wind madly, it all builds up in the middle, you can't get it all onto the reel, and it won't pull out nicely next time you need it. Same thing with your SM thread.
Polyester and silky embroidery threads will be worse too as they are silky, so if the thread hasn't been wound on smoothly and under tension, then it will "collapse" with gravity, then when you use the bobbin, the thread is going to be caught on itself, will feed unevenly and be stretched, then loose, giving you less than perfect stitch tension.
My other bobbin tip is store the bobbins in a plastic bobbin tray so they are lying on their edges and under a cover. Keeps them neater and they are less likely to unwind stray threads around your sewing cabinet if you store them on the spool pins build onto the cabinet door - and it keeps the thread dust free. But I do not keep thread on bobbins for long, prefer to wind a fresh one off a new spool when I start a project and can usually complete a garment with a 3/4 filled bobbin, use the remaining few metres for handsewing, then junk the rest. Then I put the thread spool that is left back into a sealed takeway container to keep it away from UV, dust and moisture.
I hope this assists you with your machine and certainly if this doesn't resolve the bobbin issues, then I'd suggest you visit your dealer and ask them to demonstrate the technique on your machine to see if there is a technical issue with it.
